1. Field of the Invention
The application relates to a method of foaming injection molding, and more particularly to a method of improving the appearance of foaming injection molding product.
2. Related Art
An increasing number of foaming injection molding product are applied to automotive interior product to achieve the light weight effect and meet the desired stiffness of product.
Chemical foaming refers to mixing a certain proportion of chemical foaming agent into resin, then injecting them into a mold cavity in closed state after melting and blending by screw of injection molding machine, the contacting surfaces of the molten resin and the mold are solidified to form a epidermal layer of product, but the central region of product remains stay at molten resin state. After mold filling of resin completed, opening the mold with a certain distance, such that the uncured molten resin in the central region along the wall thickness direction of product expands and foams to a desired thickness, then cooling and curing are carried on. In the process of chemical foaming injection molding, when the molten resin that contains foaming agent is injected between the core surface a and the cavity surface b of the mold, the bubbles produced by the foaming agent decomposition in the molten resin of the interior of the mold cavity begin to foam and grow rapidly due to the sharply decreasing gas pressure, especially serious at the end of the material flow and the surface of product. The bubbles will definitely break when they expand to a certain degree, thereby irregularity and air mark are formed on the surface of product which result in the decline of the appearance quality of product, as shown in FIG. 1, thus, it has to apply additional secondary processing to reprocess the appearance of product.
As mentioned above, it is easy to form irregularity and air marks due to the broken bubbles on the surface of product produced by the related process of chemical foaming injection molding technology, and finally result in the appearance defect.
In addition, because the air pressure inflated into the closed mold cavity is often below 2 MPa in prior art, specific leakproof structures and an air pressure detection and control system are essential for the mold. Meanwhile, the molds with the specific leakproof structures and the ordinary injection molds are used for their predetermined special applications and not in a common use, thus the cost for the molds is increased and the applications are limited.
Thus, a method to improve the appearance of foaming injection molding product is needed.